schema-schema: add "stringprefix" to union strategies.

Absence of this is an oversight, and I just happened to catch it while
passing through the vicinity.

Also: dropped a comment for later review on the bytesprefix strategy.
While adding the stringprefix strategy, it's hard not to notice that
variable length strings are allowed; so, it occurs to me we should
probably do the same for byteprefix.  (Also, possibly renaming it to
byte*s*prefix.)  Doing this would also fix the ancient weirdness of
the map being flipped in an awkward way to evade int keys, which is
a very happy coincidence (and in retrospect, I'm not sure why we didn't
think of this solution earlier).

schema/gen/go/_demo/omg-schemas/schemaschema_test.go

@@ -193,14 +193,16 @@ func init() {
 			"UnionRepresentation_Keyed",
 			"UnionRepresentation_Envelope",
 			"UnionRepresentation_Inline",
+			"UnionRepresentation_StringPrefix",
 			"UnionRepresentation_BytePrefix",
 		},
 		schema.SpawnUnionRepresentationKeyed(map[string]schema.TypeName{
-			"kinded":     "UnionRepresentation_Kinded",
-			"keyed":      "UnionRepresentation_Keyed",
-			"envelope":   "UnionRepresentation_Envelope",
-			"inline":     "UnionRepresentation_Inline",
-			"byteprefix": "UnionRepresentation_BytePrefix",
+			"kinded":       "UnionRepresentation_Kinded",
+			"keyed":        "UnionRepresentation_Keyed",
+			"envelope":     "UnionRepresentation_Envelope",
+			"inline":       "UnionRepresentation_Inline",
+			"stringprefix": "UnionRepresentation_StringPrefix",
+			"byteprefix":   "UnionRepresentation_BytePrefix",
 		}),
 	))
 	ts.Accumulate(schema.SpawnMap("UnionRepresentation_Kinded",
@@ -224,8 +226,15 @@ func init() {
 		},
 		schema.StructRepresentation_Map{},
 	))
+	ts.Accumulate(schema.SpawnStruct("UnionRepresentation_StringPrefix",
+		[]schema.StructField{
+			schema.SpawnStructField("discriminantTable", "Map__String__TypeName", false, false), // todo: dodging inline defn's again.
+		},
+		schema.StructRepresentation_Map{},
+	))
 	ts.Accumulate(schema.SpawnStruct("UnionRepresentation_BytePrefix",
 		[]schema.StructField{
+			// REVIEW: for schema-schema overall: this is a very funny type.  Should we use strings here?  And perhaps make it use hex for maximum clarity?  This would also allow multi-byte prefixes, which would match what's already done by stringprefix representation.
 			schema.SpawnStructField("discriminantTable", "Map__TypeName__Int", false, false), // todo: dodging inline defn's again.
 		},
 		schema.StructRepresentation_Map{},